1. Field of the Invention
The present invention relates to a method for manufacturing a particulate filter comprising a membrane with a high density array of regularly spaced micropores and a macroporous support.
2. Description of the Background Art
A lithographic apparatus or device is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate, such as a membrane substrate. A lithographic device can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
The formation of membrane filters with small straight-through holes of extremely small diameter and methods of making such porous bodies and/or microporous membranes are well known. Microporous membranes can be produced with particles which produce chains of defects in the glass, crystal or polymer membrane corresponding to the path of the particles. These defects make the areas around them very sensitive to various chemical agents. An exposure of relatively short duration to these agents makes it possible to produce pores at various locations. A longer exposure makes it possible to expand the number of pores. Accordingly, in various prior art processes the duration of chemical attack or etching facilitates control of the diameter of the pores produced, i.e. the filtration characteristic of the filter.
Various methods including combinations of irradiation damage along substantially straight paths and chemical removal of the damaged material to provide pores or holes; bombarding a solid with heavy energetic particles to produce tracks of radiation damaged material which are removed by etching; forming ionization tracks in a membrane and removing by exposure to a suitable etchant solution; two-step etching processes that permits widening of the ion tracks to make a range of larger pore sizes; and/or the like. Typically the pores have been constructed to have a conical shape so as to assist back flushing.
Often, however, filters constructed as disclosed have suffered from a high or higher specific flow resistance per unit area compared to other filter technologies. As a result, larger areas and/or higher differential pressures are required to achieve a particular filtrate flow than for low resistance filters. One solution for this issue has been the use of a macroporous support for a microporous filter membrane to enable thinner filter membranes to withstand the same filtration pressure. It has been observed that flow resistance is reduced in proportion to the reduction in thickness. Likewise, Keping Han, Wendong Xu, Ariel Ruiz, Paul Ruchhoeft, and Shankar Chellam, “Fabrication and Characterization of Polymeric Microfiltration Membranes using Aperture Array Lithography,” Journal of Membrane Science, Vol. 249 Issues 1-2, pages 193-206 (2005) discloses the use of a regular array of etched pores to form a filter membrane. This enables higher pore densities than are possible by the random array formed by ionization tracks because it eliminates the possibility of overlapping pores which compromise the selectivity of the filter. The flow resistance is reduced in inverse proportion to the increase in pore density. Primarily, the filters with a support have been formed on a flat solid substrate, thereby having at least the additional material cost of the flat substrates, multiple fabrication process steps, with the attendant inherent variation, and the fabrication of a large area of membranes by splicing or tiling together a multitude of small discrete membranes.
Accordingly, it would be beneficial to the art field of producing filter membranes to accomplish at least one of minimizing process variation through producing the filter membranes in a continuous process; producing filter membranes with increased and/or enhanced pore density; producing filter membranes without the necessity of a solid support; and/or the like.